1. Field of the Invention
The present invention relates to an objective lens system of a telescope, used (for example) in an astronomical telescope and the like, which exhibits optimum optical performance both in an observing mode through the eyepiece, and in a photographing mode to photograph a target object.
2. Description of the Related Art
An optical system, for example, an objective lens system for an astronomical telescope, including a combination of a positive lens element and a negative lens element made of glass materials having different dispersion values, has been widely known and used. An objective lens system of a telescope in general has a long focal length, and it is therefore necessary to correct axial chromatic aberration. In particular, it is essential to correct the secondary spectrum of axial chromatic aberrations at a higher magnification in the observing mode through the eyepiece. In order to correct the above-mentioned secondary spectrum of axial chromatic aberration, there have recently been many attempts to employ fluorite and low dispersion special glass to form a positive lens element. However, these prior arts have caused an increase in cost because a positive lens element in the first lens group having a large-diameter has to be made of an expensive glass material such as fluorite or low dispersion special glass and the like.
On the other hand, along with an increase in popularity of astronomical photography, an objective lens system of an astronomical telescope has been required to have a wider angle-of-view. In connection with this trend, an objective lens system disclosed in Japanese Unexamined Patent Publication No. Sho-57-93309 should be noted as an example. According to this patent publication, by attaching a detachable field-curvature correcting optical system to an objective lens system of an astronomical telescope in which axial chromatic aberrations are sufficiently corrected, the astronomical telescope can be used in both the observing mode through the eyepiece and the photographing mode to photograph a target object. More specifically, a wider angle-of-view required for the photographing mode is attained by positioning a detachable field-curvature correcting lens element in the vicinity of the image plane of the first lens group in which axial chromatic aberrations are sufficiently corrected by the combination of a negative lens element and a positive lens element.
However, the above-explained example has also caused an increase in cost because the field-curvature correcting lens element has to additionally be attached to the first lens group after the axial chromatic aberrations have been sufficiently corrected.